Many electronic devices include multiple electronic components attached to a substrate, such as a printed circuit board (PCB). The substrate provides physical support to electronic components including integrated circuits, electronic subassemblies, capacitors, resistors and similar devices, and provides connection paths to electrically connect the components to form electrical circuits which are required for a functioning electronic device.
Electronic devices may fail to function if their electronic components become damaged, such as by exposure to dust or water. Water is particularly harmful for electronic devices as electronic components are likely to electrically short or malfunction after brief exposure to liquids or moisture. Specifically, exposed metal areas having voltage differentials in close proximity can easily experience short circuit events when corrosion or water immersion bridges the gap between such areas. The electronic components of electronic devices may be surface treated with a protective material, such as a film, to provide mechanical, electro-magnetic, and/or chemical protection. For example, electronic components may be surface treated with an epoxy-based compound to provide protection from environmental hazards.
Many electronic devices also include external components to protect the active electronic elements and ensure the functionality of the device. The external components may be designed to be a permanent part of the electronic device, or may be replaceable. External components include items such as lenses, lens covers or liners, antenna shields, thermal spreaders and similar components and may be made of glass, sapphire, plastic, or other materials with specific electro-magnetic properties. These external components may be surface treated to provide mechanical, electro-magnetic, and/or chemical protection.
For example, a surface mount connector may have multiple leads that physically and electrically couple the connector to a printed circuit board. The body of the connector may be positioned off the printed circuit board while the leads are attached to bonding pads on the circuit board. A film may be used to seal the interface between the connector and the printed circuit board to prevent the ingress of moisture or various chemicals and allow the connector to remain mechanically compliant with the circuit board. Similarly, a mechanically tough external substrate may be coated with a film to prevent scratching, resist abrasive environments or bleed static charge accumulation.
The International Protection Marking (IEC standard 60529), also known as the IP Code, classifies and rates the degree of protection provided against intrusion (body parts such as hands and fingers), dust, accidental contact and water by mechanical casings and electrical enclosures. An IP code is expressed as IP##, with the first number referring to protection from solid particles and the second digit referring to protection from ingress of water. For water protection, level 0 is not protected from water, level 1 is protected from dripping water, level 2 is protected from dripping water when a device is at 15° tilt, level 3 is protected from spraying water, level 4 is protected from splashing of water, level 5 is protected from water jets, level 6 is protected from powerful water jets, level 6K is protected from powerful water jets with increased pressure, level 7 is protected from immersion in water up to 1 m of water for 30 minutes, level 8 is protected from continuous immersion in water beyond 1 m and level 9 is protected from powerful high temperature water jets. A value of “x” indicates that there is no data for the amount of protection provided.
A number of different films and treatment techniques have been developed to protect electronic devices from damage. For example, U.S. Pat. No. 8,492,898 to Ferdinandi et al., U.S. Pat. No. 8,163,356 to Coulson, and U.S. Pat. App. Pub. 2013/0240256 to Von Werne are directed to polymeric films for electronic devices to protect from water damage. These references describe the application of a hydrophobic polymeric film having a thickness between at least one to ten microns (1-10 μm) around electronic components on substrate assemblies. The film may be deposited by plasma-assisted chemical vapor deposition of fluorohydrocarbon monomers onto at least part of the substrate surface. The plasma process applies charges to the surface of the substrate so molecular fragments of the fluorohydrocarbon monomers created by the plasma, or pulsed plasma, may be bonded to the charged surface. These methods for applying a film involve placing the electronic component or substrate to be coated in a vacuum chamber having a pressure of about 10 to 200 torr. The vacuum chamber is filled with a gaseous or liquid fluorohydrocarbon monomer and a voltage is applied, creating plasma. Plasma constituents settle on and adhere to the electronic component or substrate, providing a water-resistant film on the surface.
Current commercially available protective films suffer a number of drawbacks. The large size of the fluorohydrocarbon monomers prevents them from diffusing through the entire reticulated structure of the substrate assemblies of electronic devices. The plasma deposition process also creates a distribution of molecular fragments from the fluorohydrocarbon monomer that do not readily wet the surfaces of the substrate assemblies. A failure to wet the surface of a substrate results in the formation of islands of film and incomplete encapsulation of the substrate assemblies. The process also forms hydrogen fluoride byproducts. Most current films are at best IPx4 compliant and only provide protection from splashing of water.
In addition, current films often result in treatment-induced high impedance, open circuit or intermittent function of movable electrical contacts. These treatment-induced issues may result in component and system level functional failure of the electronic device (often termed “out of box failures.”) The root cause of these failures is contamination of the electrical contact zone from the films. These problems occur especially when the film thickness becomes greater than 500 nm (0.5 μm) and when large molecular weight films are used, such as parylene and cross-linked fluoroacrylates.